Light in nature covers an enormous range of luminance levels, from starlight to bright sunlight. Yet traditional imaging technologies, both digital and analog, offer diminished experience because they cannot parallel the wide range of luminance and contrast that is visible to our eyes. In response, HDR technologies are being developed to allow an extended range of color, luminance and contrast to be displayed.
HDR technologies focus on capturing, processing and displaying content of a dynamic range wider than SDR technologies. Today, HDR capabilities of HDR displays may vary, notably according to their maximum peak white luminance: some HDR display may offer 500 nits as other one may reach 1000 nits or even 1500 nits.
Although HDR displays and cameras to capture HDR content are presently in development, HDR content need to undergo HDR to LDR tone mapping for legacy displays to be able to reproduce the content. Similarly, a HDR content prepared for instance for a 1000 nits HDR display need to undergo HDR to “lower-HDR” tone mapping for instance for a 500 nits HDR display to be able to reproduce the content. There is therefore a need for HDR to “lower-HDR” tone mapping.
Tone reproduction, also known as tone mapping, aims to map an image's original range of luminance values to a lower range of luminance values that can be reproduced by a display. Often, but not always, tone mapping is carried out on a luminance channel that is derived from an input HDR color image. The output of the tone mapping can be recombined with the color information retained from the input original HDR image, so that a new output color image is produced with a HDR dynamic range lower than the input HDR image.
Tone mapping algorithms can be classified into two broad classes. A first class can be defined as global tone mapping. This involves applying compressive tone mapping function(s) (e.g., sigmoidal functions or logarithmic functions) independently to the luminance values of each pixel of the image or image sequence. A second class can be defined as local tone mapping (also known as spatially varying tone mapping). Local tone mapping takes into account, for each pixel, the luminance value of that pixel, as well as information from its neighboring pixels.
When the range of luminance values of an input image is reduced, it is usually reduced by a very large amount, after the use of a tone mapping operator adapted for HDR to LDR conversion. For example, existing tone mapping HDR to LDR operators compress full luminance channel information (e.g., maximum 4000 nits content) to fit into the very low ranges of the legacy LDR displays (e.g., 100 nits content). On the opposite, a HDR to lower-HDR tone mapping operator may be required to reduce the range by a significantly smaller factor, e.g., (from maximum 4000 nits to 1000 nits for range compression between different HDR displays). The existing HDR to lower-HDR tone mapping operators fail to fully reproduce the HDR sensation because they compress the whole range of luminance and rescale linearly to fit into the target display luminance range, resulting instead in a sensation of LDR content. Another problem is that existing HDR to lower-HDR tone mapping operators may also reduce contrast in highlight pixels, the effect of which may be especially noticeable when the highlights are a prevalent part of the picture.